Various kinds of conventional articles which are liquid at ordinary temperature, among cosmetics, deodorants, bath additives, fragrances, deodorizers, foods, medicines, and the like, contain oil-soluble substances, such as colorants, flavoring materials, vitamins, antioxidants, preservatives, bactericides, oils, fats, etc., solubilized or dispersed in an aqueous medium by the action of emulsifying agents. However, these articles have a problem that the oil-soluble substances solubilized or dispersed in an aqueous medium with emulsifying agents separate out from the aqueous medium during preparation of the articles or during storage or transportation thereof after their preparation.
Most of the techniques proposed so far for eliminating such oil/water separation problem are to add polyhydric alcohols, surfactants, or the like as solubilizing or dispersing agents. Some of such techniques have come into practical use. Representative examples of the solubilizing or dispersing agents proposed so far include lecithin and surfactants such as polyoxyethylene sorbitan monooleate (nonionic surfactant), ethylene oxide adducts of hydrogenated castor oil, and sucrose fatty acid esters.
Among the conventional solubilizing or dispersing agents, lecithin is extensively used because it is a highly safe substance derived from a natural substance such as egg yolk or soybean. However, since lecithin has insufficient hydrophilicity, use of lecithin for the solubilization or dispersion of oil-soluble substances in aqueous mediums necessitates use of water-soluble solvents, e.g., polyhydric alcohols. If the final product is a food, the additional use of such water-soluble solvents causes problems concerning taste. Another disadvantage of lecithin is that, since it is derived from a natural substance, it is susceptible to putrefaction, rancidity, or the like and thus lacks stability.
Polyoxyethylene-based nonionic surfactants have a disadvantage that the ethylene oxide chain contained as a hydrophilic group in the molecular chain thereof degrades with the lapse of time, resulting in reduced solubilizing or dispersing power. In addition, there is a fear that the degradation of those surfactants may result in formation of formalin, which is a decomposition product of ethylene oxide chain, or in a decrease in pH. Therefore, use of the polyoxyethylene-based nonionic surfactants as food additives is prohibited in Japan from the standpoint of safety. Thus, those surfactants are restricted in their applications. Sucrose fatty acid esters may degrade under acidic conditions, so that they have a problem that they cannot exhibit sufficient solubilizing or dispersing power under low-pH conditions.
On the other hand, solubilizing or dispersing agents comprising polyglycerol fatty acid ester (hereinafter often referred to as PoGE), which are emulsifying agents whose use as a food additive has been authorized, have been proposed (JP-A-61-234920 and JP-A-62-250941). (The term "JP-A" as used herein means an "unexamined published Japanese patent application.") However, use of the proposed solubilizing or dispersing agents comprising PoGE for the solubilization or dispersion of oil-soluble substances in aqueous mediums disadvantageously necessitates additional use of another additive such as polyhydric alcohols. Further, the fatty acid moieties of the PoGE are mainly derived from an unsaturated fatty acid having low long-term stability. Thus, the proposed PoGE-based solubilizing or dispersing agents have problems that taste is influenced by the additives when the article is a food or the like, and that the poor long-term stability influences the distribution and storage of the article.
In addition, use of the conventional solubilizing or dispersing agents in the solubilization or dispersion of oil-soluble substances has disadvantages concerning article production. For example, for solubilizing or dispersing oil-soluble substances in aqueous mediums without fail, it is necessary to employ emulsifiers, high-pressure homogenizers, or the like for the solubilizing or dispersing operation to apply a powerful shearing force to a raw-material mixture. Consequently, there has been a desire for the development of solubilization or dispersion systems which comprise aqueous mediums and oil-soluble substances easily solubilized or dispersed therein, and which are stable over a prolonged period of time and do not influence taste, etc. in food applications.